The inventor is an electrician who has been involved in residential construction and remodelling. When the need arises to enlarge an existing hole some means of maintaining the pilot drill in the center of the existing hole must be provided for.
There are a number of relatively elaborate devices to center a pilot drill in an existing hole. Compared to a simple disc, which can only partially slide into an existing hole, these are expensive to manufacture and heavy to carry around. The problem with a simple disc however, is that if it is used to enlarge an existing hole in a wall--once the hole is cut, the disc, together with the round cut out piece may fall down into the wall where it cannot be recovered. Another problem with a simple disc is that it is difficult to hold in an existing hole while cutting. When a large diameter hole is being cut it is common for the cutting teeth to catch in the workpiece twisting the drill out of axial alignment in the hands of the user. When cutting in wood it is not uncommon to break pilot bits. When cutting in sheetrock the result is an enlarged pilot hole and a roughly cut hole.
U.S. Pat. No. 5,871,310 issued to R. Mortensen discloses the use of a tapered dowel plug. One problem with the tapered dowel plug is that it may vibrate loose in sheetrock or other workpieces. In other workpieces if it is able to be jammed into the existing hole sufficiently so that it does not vibrate or wrench free during the cutting process, then it must be subsequently removed for reuse. Yet another problem is that the tapered dowel plug, together with the cutout piece, may fall into the interior of a wall where it cannot be recovered.